The overall objective of this application is to identify the transcriptiona mechanisms that control gene expression in neurons. The long-term goals of these studies is to be able to selectively regulate neuropeptide expression and to identify the developmental mechanisms that control neuronal phenotypes. As a model system, the neuronal control of calcitonin/calcitonin gene-related peptide (CT/CGRP) transcription will be investigated. The foundation for this application is the recent finding tha the CT/CGRP enhancer is controlled in neural-like thyroid C-cells by a synergistic interaction between cell-specific HLH and octamer proteins that is repressed by the glucocorticoid and retinoic acid receptors. While HLH and octamer proteins are key transcriptional regulators in other systems, a present CT/CGRP is the only example of HLH-octamer synergism and, as such, provides a unique system for studying interactions between these factors. Since transcriptional synergy is an increasingly common theme, it seems likely that combinatorial codes of these factors will be used by other gene in the nervous system. The experiments in this application will first identify candidate octamer proteins using PCR and expression screening approaches. Both new and established octamer proteins will be tested for interactions with HLH proteins using binding and transactivation assays. Repression of HLH-octamer synergism by members of the steroid/retinoid receptor family will then be pursued by testing receptor binding to octamer proteins. This type of repression appears to be an emerging theme of counteractive gene regulation. Finally, experiments will test the cell- specificity of the HLH-octamer enhancer and address the possibility of a CNS-specific enhancer using a neuronal cell line and a series of constructs in transgenic mice. Identification of distinct enhancers for CT/CGRP expression will facilitate targeted and selective control of CT and CGRP levels. This may be therapeutically useful for treating certain disorders such as osteoporosis and hypertension. Furthermore, understanding -the regulation of HLH-octamer activity will potentially shed light on the wide ranging effects of glucocorticoids and retinoic acid on the development and function of the nervous system.